LONDON, England (CNN) -- A meningitis outbreak is threatening the lives of tens of thousands of people in Darfur, according to an aid agency expelled from the country last week.

A Doctors without Borders medic helps a sick child in a Darfur refugee camp.

Humanitarian organization Doctors Without Borders, also known as Médecins Sans Frontières (MSF), says it was preparing to vaccinate around 100,000 people in southern and western Darfur against meningitis, when it was told to leave the country.

"We were in the process of organizing a vaccination campaign. At the moment there's no one there that can take over those activities, which is really worrying," Gemma Davies of MSF told CNN.

The country's Ministry of Health officially declared a meningitis outbreak on March 2 at the Kalma Camp, which shelters more than 90,000 refugees in southern Darfur.

"Living in such close proximity, the potential for this outbreak to spread quickly is quite high, Davies, MSF project coordinator for South Darfur, told CNN.

"With no health care providers to give vaccinations, and with no one to manage the meningitis cases when they come, it's really concerning," she added.

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The decision by the Sudanese government to throw out the aid groups came a day after the International Criminal Court at The Hague issued an arrest warrant for Sudanese President Omar Hassan al-Bashir for war crimes and crimes against humanity in Darfur.

However, not all international aid organizations are affected. Others including World Vision international continues its work, which reaches approximately 500,000 internally displaced people in South Sudan.

The U.N. has received reports that international staff members of five non-governmental organizations had been detained for up to four hours.

Speaking on the phone from Nairobi, an aid worker, who asked not to be identified for fear of further harassment, told CNN that his colleagues who remained in Khartoum were being intimidated and threatened by government agents.

In some cases Sudanese government representatives "had started gathering banking details and confiscating computers, communications equipment and vehicles from the NGOs", a U.N spokesman said at a news conference last week.

Last week, the U.N. warned that the loss of NGOs in Sudan would cause 1.1 million people to go without food aid and health care, and more than 1 million to have no access to water.

Oxfam, whose work focuses on providing safe drinking water, was one of the aid groups who were told their registration to operate in Sudan was no longer valid. Ninety percent of their staff are Sudanese nationals, who mostly remain in country, unable to continue their relief work under threat of arrest.

"These agencies are vital implementation partners for the United Nations and account for at least half of the humanitarian capacity in Darfur," said Catherine Bragg, U.N. Deputy Emergency Relief Coordinator at the news conference in New York.